This invention relates to an electronic measuring or monitoring device for assessing the ability of a storage battery to deliver power to a load. More specifically, it relates to improved apparatus of the type disclosed previously in U.S. Pat. Nos. 3,873,911, 3,909,708, 4,816,768, 4,825,170, 4,881,038, and 4,912,416 issued to Keith S. Champlin.
Storage batteries are employed in many applications requiring electrical energy to be retained for later use. Most commonly, they are employed in motor vehicles utilizing internal combustion engines. In such applications, energy stored by "charging" the battery during engine operation is later used to power lights, radio, and other electrical apparatus when the engine is stopped. The most severe demand upon the battery of a motor vehicle is usually made by the starter motor. Failure to supply the starter motor with sufficient power to crank the engine, particularly in cold weather, is often the first indication of battery deterioration. Clearly, a simple measurement that accurately assesses a battery's ability to supply cranking power is of considerable value.
Prior to the introduction of the dynamic conductance testing method disclosed in the six U.S. patents enumerated above, the method most generally available for assessing a battery's ability to supply cranking power was the standard load test. This test subjects a battery to a heavy dc current having a predetermined value dictated by the battery's rating. After a prescribed time interval, the battery's voltage under load is observed. The battery is then considered to have "passed" or "failed" the load test according to whether its voltage under load is greater, or less, than a particular value.
Although the standard load test has been widely used for many years, it has several serious disadvantages. These include:
1. The test draws a large current and therefore requires apparatus that is heavy and cumbersome.
2. Considerable "sparking" can occur if the test apparatus is connected or disconnected under load conditions. Such "sparking" in the presence of battery gasses can cause an explosion with the potential for serious injury to the user.
3. A standard load test leaves the battery in a significantly reduced state-of-charge and therefore less capable of cranking the engine than before the test was performed.
4. The battery's terminal voltage decreases with time during performance of the load test. Accordingly, test results are generally imprecise and often dependent upon the skill of the operator.
5. Load test results are not repeatable since the test itself temporarily polarizes the battery. Such test-induced polarization significantly alters the initial conditions of any subsequently-performed tests.
A practical alternative to the standard load test is taught in the six U.S. Patents enumerated above. These documents disclose electronic apparatus for accurately assessing a battery's condition by means of small-signal ac measurements of its dynamic conductance. They teach that a battery's dynamic conductance is directly proportional to its dynamic power--the maximum power that the battery can deliver to a load. Dynamic conductance is therefore a direct measure of a battery's ability to supply cranking power. In comparison with the load test method of battery appraisal, the dynamic conductance testing method has many advantages. For example, dynamic conductance testing utilizes electronic apparatus that is small and lightweight, draws very little current, produces virtually no sparking when connected or disconnected, does not significantly discharge or polarize the battery, and yields accurate, highly reproducible, test results. Virtually millions of battery measurements performed over the years have fully corroborated these teachings and have proven the validity of this alternative testing method.
One disadvantage, however, of the dynamic conductance testing method has been the fact that test results are somewhat dependent upon the battery's state-of-charge. Accordingly, the methods and apparatus disclosed in the first five of the six U.S. patents cited above have generally required that the battery be essentially fully charged to be tested. Since many batteries are, in fact, fairly discharged when they are returned for replacement under warranty, or when they are otherwise suspected of being faulty, it has been frequently necessary to recharge a battery before testing it. Such recharging is costly and time-consuming. Clearly, a simple method for performing accurate dynamic conductance tests on batteries "as is"--batteries that may be only partially charged--would be of considerable benefit.
Great progress toward solving this problem has been engendered by the methods and apparatus disclosed in the sixth U.S. patent cited above; U.S. Pat. No. 4,912,416. As is well known to those skilled in the art, a battery's state-of-charge is directly related to its open-circuit (unloaded) terminal voltage. By utilizing this fact, along with extensive experimental data, an empirical relationship was established between a battery's state-of charge, as reflected by its open-circuit voltage, and its relative dynamic conductance, normalized with respect to its fully-charged value. This empirical relationship was first disclosed in U.S. Pat. No. 4,912,416. Further, apparatus disclosed therein utilized this empirical relationship, along with measurements of open-circuit voltage, to appropriately correct dynamic conductance readings--thus yielding battery assessments that were essentially independent of the battery's state-of-charge.
However, the measuring apparatus disclosed in U.S. Pat. No. 4,912,416 utilized an inconvenient two-step testing procedure requiring intermediate interaction by the user. The battery's open-circuit voltage was first measured. Next, using the results of the voltage measurement, the user adjusted a variable attenuator to an appropriate setting. Finally, the dynamic conductance was measured. By virtue of the previously adjusted variable attenuator, the quantitative or qualitative dynamic conductance information ultimately displayed to the user conformed with that of a fully-charged battery even though the battery may, in actual fact, have been only partially charged when tested.
The state-of-charge problem was thus solved in principle by the methods and apparatus taught in U.S. Pat. No. 4,912,416. The required procedure was somewhat inconvenient, however. It is therefore quite apparent that improved apparatus which provides automatic state-of-charge correction--a correction not requiring intermediate interaction by the user--would be highly advantageous. Just such improved electronic battery testing apparatus, providing automatic compensation for low state-of-charge, is disclosed herein below.